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Encyclopedia > Gravimetry

Gravimetry is the measurement of a gravitational field. Gravimetry may be used when either the magnitude of gravitational field or the properties of matter responsible for its creation are of interest. Gravity is a force of attraction that acts between bodies that have mass. ...

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Units of measurement

Gravity is usually measured in units of acceleration. In the SI system of units, the standard unit of acceleration is 1 metre per second squared (abbreviated as m/s2). Other units include the gal (sometimes known as a galileo, in either case with symbol Gal), which equals 1 centimetre per second squared, and the gee (gn), equal to 9.80665 m/s2. The value of the gn approximately equals the acceleration due to gravity at the Earth's surface (although the actual acceleration g varies fractionally from place to place). Acceleration is the time rate of change of velocity, and at any point on a v-t graph, it is given by the slope of the tangent to that point In physics, acceleration (symbol: a) is defined as the rate of change (or time derivative) of velocity. ... Cover of brochure The International System of Units. ... Metres per second squared is the SI derived unit of acceleration (scalar) and (vector), defined by distance in metres divided by time in seconds and again divided by time in seconds. ... The galileo or gal is the CGS unit of acceleration. ... The metre, or meter (symbol: m) is the SI base unit of length. ... g (also gee, g-force or g-load) is a non-SI unit of acceleration defined as exactly 9. ...


How gravity is measured

An instrument used to measure gravity is known as a gravimeter, or gravitometer. Since general relativity regards the effects of gravity as indistinguishable from the effects of acceleration, gravimeters may be regarded as special purpose accelerometers. Many weighing scales may be regarded as simple gravimeters. In one common form, a spring is used to counteract the force of gravity pulling on an object. The change in length of the spring may be calibrated to the force required to balance the gravitational pull. The resulting measurement may be made in units of force (such as the Newton), but is more commonly made in units of Gals. The term gravitometer is sometimes used synonymously with the more appropriate term gravimeter - a device designed to measure either the local gravitational field, or changes in the gravitational field. ... General relativity (GR) is the geometrical theory of gravitation published by Albert Einstein in 1915. ... Acceleration is the time rate of change of velocity, and at any point on a v-t graph, it is given by the slope of the tangent to that point In physics, acceleration (symbol: a) is defined as the rate of change (or time derivative) of velocity. ... A depiction of an accelerometer designed at Sandia National Laboratories. ... A doctors scale A weighing scale (usually just scale in common usage) is a device for measuring the weight of an object. ... Springs A spring is a flexible elastic object used to store mechanical energy. ... The newton (symbol: N) is the SI unit of force. ... The galileo or gal is the CGS unit of acceleration. ...


More sophisticated gravimeters are used when precise measurements are needed. When measuring the Earth's gravitational field, measurements are made to the precision of microGals to find density variations in the rocks making up the Earth. Several types of gravimeters exist for making these measurements, including some that are essentially refined versions of the spring scale described above. These measurements are used to define gravity anomalies. Gravity anomalies are widely used in geodesy and geophysics. ...


Besides precision, also stability is an important property of a gravimeter, as it allows the monitoring of gravity changes. These changes can be the result of mass displacements inside the Earth, or of vertical movements of the Earth's crust on which measurements are being made: remember that gravity decreases 0.3 mGal for every metre of height. The study of gravity changes belongs to geodynamics. In Wikipedia, precision has the following meanings: In engineering, science, industry and statistics, precision characterises the degree of mutual agreement among a series of individual measurements, values, or results - see accuracy and precision. ... The word stability has a number of technical meanings, all related to the common meaning of the word. ... Height is a measurement of the distance from the bottom to the top of something which is upright. ...


All modern gravimeters use specially-designed quartz zero-length springs to support the test mass. Zero length springs do not follow Hooke's Law, instead they have a force proportional to their length. The special property of these springs is that a vertical pendulum can be designed with a period approaching a thousand seconds. This detunes the test mass from most local vibration and mechanical noise, increasing the sensitivity and utility of the gravimeter. The springs are quartz so that magnetic and electric fields do not affect measurements. The test mass is sealed in an air-tight container so that tiny changes of barometric pressure from blowing wind and other weather do not change the buoyancy of the test mass in air. Quartz is amongst one of the most common minerals in the Earths continental crust. ... A zero-length spring has a physical length equal to its stretched length. ... In physics, Hookes law of elasticity is an approximation which states that if a spring is elongated by some distance, x, the restoring force exerted by the spring, F, is proportional to x by a constant factor, k. ...


Gravimeters have been designed to mount in vehicles, including aircraft and ships. These special gravimeters isolate acceleration from the movement of the vehicle, and subtract it from measurements. The acceleration of the vehicles is often hundreds or thousands of times stronger than the changes being measured.


History

The modern gravimeter was developed by Lucien LaCoste and Arnold Romberg in 1936. LaCoste, Lucien, 1908-1995, scientist, inventor Many other articles will tell you the important things about Dr. LaCoste, but the purpose of this entry is to pass on two memories of a great man. ...


They also invented most subsequent refinements, including the ship-mounted gravimeter, in 1965, temperature-resistant instruments for deep boreholes, and lightweight hand-carried instruments. Most of their designs remain in use (2005) with refinements in data collection and data processing.


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